Method of drying



Patented May 16, 1933 PATENT oFFicE V UNITED STATES- rmnwar a. momma, orenrcaoo, armors, assmnon we ran cano'rax contain, or cnrcaco, ILLINOIS,A ooaronurron or namwm mnaa'ruon ofmnme no Drawing.

This invention relates to a method for drying fibrous materials,particularly to the drying thereof into board-like sheets formed on amachine generally similar to machines employed in the formation ofpaper, and has for its object to provide a method which is moreeflicient in operation than thoseherev tofore proposed. a v The materialto be dried, according'to the method disclosed herein, is moreparticularly the board-like materials which are now made and sold asinsulating building boards, substantially all of which are made fromvegetative or wood fiber. In the manufacture of-boards of this type theraw material is'not reduced to a condition suitable for the manufactureof paper, but instead the reduction is to fiber bundles comprising aplurality of fibers, although incidental to such reduction "there is, ofcourse, some reduction of thematerial to elemental fibers. In the d ingof these fibrous materials, after they have been sheeted on a boardmachine from a dilute suspension thereof, the conditions'are in generalquite difierent from the conditions met in. drying paper sheets.-Thatis, it is possible to express the suspending fluid'from the sheetedstock down to a point at which there is substantially no free fluidtherein-"and, due to the fact that the fibers are hydrated but veryslightly if at all, there is little or no water of hydration to beremoved from the sheeted stock.

In the ordinary drying of paper stock there are generally considered tobe three stages of water removal ;first, the evapora- 40 tion of whatmight be termed the loosely held water, that is the water mechanicallyheld in the sheet in the larger fiber interstices; second, the removaloffi the water which is retained in the sheet due to capillarity;-andthe ,third stage comprising the of such materials after formation boardsthe raw material, whether sugar cane Application fled Aim! 5, 1930.Serial No. 442,085.

removal of the chemically or physically combined water of hydration.

It has been determined that a sheeted insulating) board formedfrom'sugar case bagasse fi er bundles has characteristics which make itparticularly adaptable to" the new method of dryin which willhereinafter be more fully descri d. In addition to the use of thismethod in connection with the drying of insulating board made from sugarcane bagasse, the method may be advantageously employed in the dryin ofother fibrous insulating boards such as t ose made from wood, straw,corn stalks, and the like although, for various reasons which will beevident, the results will not be quite as satisfacto as are obtained inthe drying of board ma efrom sugar cane bagasse,

In the manufacture of these insulating bagasse, wood, corn stalks,straw, or the like, is lightly digested or otherwise repared forseparation into fiber bund les, which separation is accomplishedpreferably by means of swing" hammer shredders or equivalent refiningmachines Due to the' fact that in the initial preparation ofthe fibrousmaterials there is, in general,' substantially nothin removed therefromexcept the water so uble materials, such fiber bundles retain aconsiderable amount of lignin and various inorganicconstituents, and inthe case of bagm and straw there is .particularly an appreciable contentof complex silica. compounds. The lignin content and these inorganiccompounds tend to make the fibers relatively stifi whereby, whencompressed, they generally tend to return to their originalconditionfwhich feature is available of to some extent in the describedmethod of drying After the fibers are sheeted from their dilutesuspension in a machine of the nature of a paper-making machine, thesheets ass through squeeze rolls, which remove m,

unsaturated gas over the sur aces of the board, water is removedtherefrom.

It has been discovered that, by the removal of substantially all of theloosely contained water from the formed sheet through the use of thepress rolls, the subsequent drying of the sheet in a tunnel driercomprisesdrying at a continuously decreasing rate of water removal andthat, due to the fact that these sheets are approximately onehalf inchthick, as the sheets are dried the exterior portions thereof serve asnon-c0nductors thereby greatly retarding the removal of moisture fromthe interior of,the sheet.

It has been determined that, for instance,

one-half of the residual moisture may be re-' moved from the sheet inthe tunnel drier in approximately the same time,as is required for theremoval of the succeeding quarter of the original moisture, and that theremoval of the next one-eighth of the residual moisture requires aboutthe same time as does the removal of the first half of the moisturecontent.

By subjecting all surfaces of the sheet in a tunnel drier to heatsufficient to remove about one-half of the moisture content which itcontained on entering the drier, and then completing the drying in apress having heated platens, such as is shown in U. S. Letters PatentNo. 984,870 granted Feb. 21, 1911 to J. A. Wheeler it has beendiscovered that the drying may be carried out more rapidly and moreeconomically. It has been found that by placing the sheets betweenheated platens having provision such as in-, terposed screens or thelike for the escape of such steam as may be formed in the course ofdryin a more rapid heat transfer may be obtained into the interior ofthe sheet, since by applying pressure to the heated platens, the boardbeing dried can be somewhat compacted, whereby the rate of heat transferis improved, and further, the. heat transfer is improved, due to theinti mate contact of the heated. platens with the opposed surfaces ofthe board being dried.

In the drying of insulating boards according to the method justdescribed it has been found, particularly in the drying of insulatingboards made from sugar cane bagasse if the boards are initially dried inthe tunnel drier to approximately 20% mois 5 ture content, thatappreciable pressure can be placed thereon by the platens of the press,whereby the final rate 'of drying may be increased and the timeof'drying decreased, but that due to the resiliency of the fibersbecause of the relatively high lignin content and residual inorganicmatter, when the pressure is released, the board tends to return to itsoriginal dimensions, and this is substantially true of insulating boardsmade from corn stalk fiber, straw, wood fiber, and the like, althoughfor certain of these materials the tendency of the sheet to return toits original dimensions after release of the pressure is less than thatof sheets made from sugar cane bagasse.

It is obvious that those skilled in the art may vary the steps andcombinations of steps constituting this method within the scope of thisinvention without departing from the spirit, thereof, and therefore itis not desired to be limited to the foregoing disclosure except as maybe demanded by the claims.

What is claimed is 1. The method of drying board composed primarily offiber bundles unsuitable for paper-making and fiber, wherein afterformation of the sheet from dilute water suspension. with subsequentremoval of its loosely held water, the sheet is dried in a tunnel drierto not less than 20% moisture content and subsequently fully driedbetween heated press platens while subjected to compressiontherebetween.

2. The method of drying fiber insulating board comprising the initialextraction of Water therefrom in a tunnel drier to a moisture content ofnot less than 20% with subsequent drying while subjected to compressionbetween heated press platens.

3. The method of drying insulating board made from fiber bundles derivedfrom su ar cane bagasse, straw, wood, and the lilre, wherein sheetedfiber bundles thereof formed from a dilute water suspension are firstsubjected to pressure between squeeze rolls for the removal of theloosely held water there- 1n, are subsequently subjected to a current.of heated unsaturated gaseous medium for the removal of approximatelyone-half of its moisture content, and finally are subjected tocompression between heated press platens for the complete removal of themoisture content thereof.

4. The method of manufacturing a boardlike product composed of fiberbundles and fiber, wherein the final stage of manufacture comprisesserial drying initially to a moisture content of not less than 20% byvapor circulation, and finally through subjection to pressure betweenheated press platens and the like.

. 5. The method of drying freshly formed fiber board comprising theinitial extraction of Water therefrom to a, moisture content of not lessthan 20% by contact with a heated gas, and the final drying thereofwhile subjected to pressure between heated surfaces. 6. The method ofdrying freshly formed 5 fiber board comprising the initial extraction ofwater therefrom by contact of all surfaces with a heated gas, and thefinal drying thereof by contact of opposed surfaces with heated memberswhile subjected to pressure therebetween.

In testimony whereof I aflix my si nature.

TREADWAY B. MUN OE.

